Field of the Invention
The present invention relates to an electrically insulating, gas-tight leadthrough for at least one electrical conductor extending through a metallic sheath of an exhaust gas system of an internal combustion engine, in particular an electrically heatable catalytic converter.
Electrically insulating, gas-tight leadthroughs are known in large numbers from the prior art. Such leadthroughs are available for the most varied purposes.
Increasingly more complex exhaust gas purification systems are employed because of increasingly stricter regulations for limiting noxious exhaust gases from motor vehicles. Besides a controlled three-way catalytic converter, electrically heatable catalytic converters are employed in particular for reducing pollutant emissions during a cold start phase of a motor vehicle. An electrically heatable honeycomb body which is suitable therefor is described in International Patent Application WO 92/02714, corresponding to U.S. Pat. No. 5,411,711. The present invention is based on that prior art and reference is made to the entire contents of that reference regarding details of the structure of electrically heatable honeycomb bodies in order to avoid repetitions. The problem connected with electrical leadthroughs in an exhaust gas system of a motor vehicle is that the leadthrough must be electrically insulating, mechanically stable, gas-tight and suitable for very high temperatures and great temperature variations. Besides an electrically insulating coating, for that purpose the prior art proposes sealing with an expanding material, for example of mica fibers. However, it is difficult to produce such seals which are actually completely gas-tight and have a long service life. In connection with exhaust gas installations having a controlled three-way catalytic converter in particular, it is important for exact control that the system be gas-tight, because otherwise the oxygen measurements performed in the interior can become distorted.
Problems of a comparable nature have basically been solved in the prior art long ago by metal-ceramic constructions, for example in connection with spark plugs. However, such compounds are elaborate in their production and therefore expensive.
However, in connection with electrically heatable catalytic converters there is an additional problem which is that they are generally produced in a brazing or sintering process, and the electrical insulation for the leadthrough should already be in place for simplifying production. That insulation therefore must resist the temperatures in the brazing or sintering process.